1. Field of the Invention
The present invention relates to a gear shift shock reducing apparatus for a hydraulic control system of an automatic transmission.
2. Description of the Prior Art
In automatic transmissions, a shift is made automatically between gears either by supplying a fluid pressure, for example, a line pressure, to a corresponding friction unit or by discharging the fluid pressure from the same as a result of an upshift or downshift operation of a gear shift valve. During a gear shifting operation, the driver feels a shock (often called as a "gear shift shock") because the gear ratio (a gear reduction ratio) to be used changes, thus changing the output torque of the transmission.
The amount of the gear shift shock is determined by the amount of the torque capacity of a friction element in relation to the amount of torque required to the friction element in accordance with the driving torque of an engine, and if the amount of the required torque to the friction element is greater than the amount of torque capacity thereof, the amount of the gear shift shock increases, while, if the required amount of torque is smaller than the amount of torque capacity, the coupling force of the friction element is insufficient, allowing the friction element to slip, thus not only preventing the transmission of the driving power without any loss, but also causing a serious and a damaging overheating of the friction element. Accordingly, it is known to design each friction element such that the torque capacity thereof is slightly larger than necessary in order to avoid these problems.
Since the automatic transmission can not avoid the above-mentioned shock, it has been the conventional practice to use an accumulator a as shown in FIG. 1 to cope with this problem. Referring to the accumulator a, if a shift valve is actuated to start feeding a line pressure P.sub.L through an orifice c to a friction element d, the operating oil pressure for said friction element P.sub.A is fed to a chamber e. The accumulator a is fed with the line pressure P.sub.L within a chamber f, wherein an accumulator piston g receives a downward force by the oil pressure P.sub.A within the chamber e (pressure receiving area A) and an upward force by the oil pressure P.sub.L within the chamber f (pressure receiving area B). The piston g further receives an upward force by a spring h, thus a equilibrium equation of the forces acting upon the piston g is expressed by the following equation if a spring force by the spring h is represented by F.sub.s. EQU P.sub.A .times.A=P.sub.L .times.B+F.sub.s EQU P.sub.A =(B/A)P.sub.L +(Fs/A) (1)
Explaining now a change in the oil pressure P.sub.A vs. time (a change in the torque capacity of the friction element d), as is apparently shown together with a change in an output axle torque of a transmission vs. time in FIG. 2, when the line pressure is fed to the friction element d after the shift valve b is actuated, the oil pressure P.sub.A increases to P'.sub.A during a period beginning at the initiation of coupling and ending at the initiation of the subsequent gear change because of a sliding resistance in the friction element. Subsequently, when the gear change begins to take place as a result of the coupling of the friction element, the oil pressure P".sub.A increases up to P.sub.A" because of the occurrence of a reaction at this time. The oil pressure P.sub.A begins to move the piston g downwardly against the pressure within the chamber f and the spring force Fs of the spring h. The oil pressure value P".sub.A is expressed using said equation (1) as follows: EQU P".sub.A =(B/A)P.sub.L +(Fs/A) (2)
Since A&gt;B and the spring force Fs is small, the oil pressure P".sub.A takes a value that is a reduced value from the line pressure P.sub.L by a constant rate and since this reduced pressure is fed to the friction element d, the torque capacity of the friction element is initially suppressed small. During the downward movement of the piston g wherein the torque capacity of the friction element d is suppressed small, said friction element completes its coupling action and thereafter the piston g reaches its lower limit position. When the piston g has reached the low limit position, the accumulator stops effecting said pressure reducing function, thus allowing the oil pressure P.sub.A to increase to the same value as that of the line pressure P.sub.L.
In the above-mentioned manner, the accumulator a regulates the actuating oil pressure P.sub.A to provide a torque capacity which varies in agreement with the required torque of the friction element d that is shown by the one dot chain line in FIG. 2 although slightly larger than the latter, thus decreasing a gear shift shock without causing the occurrence of a slip of the friction element.
However, the accumulator a is relatively bulky, which measures, in diameter, 30.about.35 mm and, in length, 60.about.65 mm, as compared to the shift valve b which measures, in diameter, about 10.about.15 mm, thus making it difficult to arrange the accumulator within a limited space provided by an automatic transmission, thus causing a bulky size of a hydraulic control portion of the automatic transmission. Besides, where the accumulator a is to be used, when a release of the friction element d is necessary in response to an inoperative position (a downshifted position) of the shift valve b, this release must be effected quickly. Thus, in order to disable the function of the orifice c upon release of the friction element d, a one-way valve as shown b i in FIG. 1 has been necessiated which prevents a flow of oil from the shift valve b toward the friction element d, thus causing a complicated and expensive structure of the automatic transmission as a result of an increase in the number of component parts.
Further, the accumulator a performs the before-mentioned pressure reducing function even under a kickdown condition (a condition when an accelerator pedal is fully depressed) in the same manner as its function under normal conditions, thus providing inconveniences which will be hereinafter explained. FIG. 3 shows a typical example of a shift pattern of an automatic transmission, and as will be apparently understood from this figure, shift points under a kickdown condition are shifted toward a higher vehicle speed side as compared to the corresponding shift points under normal conditions. It is to be noted that because the engine speed is high, the required torque capacity to the friction element should be high upon making a shift at high vehicle speeds as compared to the required capacity upon making a shift under low vehicle speeds even if a driving torque is the same, so that unless the torque capacity is large enough for the required torque capacity, a slippage time period of the friction element prolongs, causing an excessive wear of the friction element during a running condition wherein a kickdown occurs frequently, resulting in a braking. However, the above-mentioned accumulator performs the pressure reducing function not only under normal condition but also under a kickdown condition in the same manner, thus running short of a torque capacity of the friction element under the kickdown condition, thus failing to meet the demanded torque capacity as mentioned above.